One type of non-volatile memory known in the art relies on magnetic memory cells. These devices, known as magnetic random access memory (MRAM) devices, include an array of magnetic memory cells. The magnetic memory cells can be of different types, for example, magnetic tunnel junction (MTJ) memory cells or giant magnetoresistive (GMR) memory cells.
Generally, a magnetic memory cell includes a layer of magnetic film in which the orientation of magnetization is alterable, and a layer of magnetic film in which the orientation of magnetization may be fixed or “pinned” in a particular direction. The magnetic film having alterable magnetization is referred to as a sense layer or data storage layer. The magnetic film that is fixed is referred to as a reference layer or pinned layer.
Conductive traces referred to as word lines and bit lines are routed across the array of memory cells. Word lines extend along rows of the memory cells and bit lines extend along columns of the memory cells. A memory cell stores a bit of information as an orientation of magnetization in its sense layer at each intersection of a word line and a bit line. The orientation of magnetization in the sense layer aligns along an axis of the sense layer referred to as its easy axis. The orientation of magnetization does not easily align along an axis orthogonal to the easy axis, referred to as the hard axis. Magnetic fields can be applied by a write circuit to flip the orientation of magnetization in the sense layer along its easy axis to either a parallel or anti-parallel orientation with respect to the orientation of magnetization in the reference layer.
To write a memory cell, the write circuit selects one word line and one bit line to set or change the orientation of magnetization in the sense layer of the memory cell. The orientation of magnetization in the sense layer of the selected memory cell is rotated in response to write currents on the selected bit line and word line. The write currents may be the same or different in magnitude, where a larger write current creates a stronger magnetic field around the write line. These write currents create magnetic fields according to the right hand rule, which act in combination to rotate the orientation of magnetization in the sense layer.
Typically, the magnetization pattern in the sense layer includes magnetization in an interior region and magnetization in edge regions. Demagnetization fields, often present in the edge regions, increase the intensity of the magnetic field needed to set or switch the state of a memory cell. In addition, as memory cells are decreased in size to increase memory device storage density, the effects of the demagnetization fields increase. The intensity of the magnetic field needed to set or switch the state of a memory cell, i.e., the coercivity, increases as the memory cell gets smaller.
The magnitude of the write currents can be increased to increase the intensity of the magnetic fields. However, larger currents and the resulting stronger magnetic fields can affect the state of adjacent memory cells in the array of memory cells. Additionally, the larger drive transistors required to drive the currents can increase the size and cost of the MRAM devices. The larger currents can also cause electro-migration problems in the word and bit lines.